Stabilization of blue-print coatings



Patented July 18, 1939 v UNITED STATES STABILIZATION OF BLUE-PRINT COATINGS Garnet Philip Ham, Old Greenwich, and Robert Bowling Barnes, Stamford, Conn., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application April 8, 1939,

" Serial No. 266,864

8 Claims.

This invention relates to stabilized blue print coating solutions and stable ferricyanide prod ucts therefor.

One of the difiiculties encountered in the preparation of blue print paper is the tendency of the paper to undergo certain changes when stored in the absence of light, which changes result in the formation of undesired blue backgrounds when the paper is flnally printed and developed. A blue background is undesirable and when the intensity becomes serious the paper is entirely unusable. For this reason, commercial blue print paper is usually not kept more than a few weeks. The factors underlying the rapid deterioration of blue print paper are quite complex. the formation of ferrous compounds in the coating, either by autoreduction or by some reaction between the ingredients of the coating material and the simultaneous presence of ferric and ferrous compounds leads to the formation of insoluble blue compounds which do not readily wash off and which produce blue backgrounds even in the portions of the print where there was no exposure to light.

In the past; it has been found advisable to use potassium ierricyanide of the greatest possible purity and even with the purest available ferricyanide, the life of blue print paper is relatively short. All attempts to substitute a cheaper ferricyanide such as, for example, sodium ferricyanide liquor or sodium ferricyanide crystals having a relatively low purity, failed because the paper produced had even less stability than ordinary blue print paper. In spite of its high cost, therefore, potassium ferricyanide of maximum purity has been standard in the blue printing art.

According to the present invention we have found that it is possible to stabilize a blue print coating so that paper coated with the stabilized solution has a much longer life than the ordinary grades of paper available at the present time. A marked advantage of the present invention lies in the fact that it is not necessary to use potassium ferricyanide of high purity; on the contrary, less pure potassium ferricyanide and even relatively impure sodium ferricyanide liquor or crystals may be employed, thereby markedly reducing the cost of the ferricyanide component to the blue print paper manufacturer.

The stabilizers of the present invention are the stable persulfates, particularly ammonium persulfate and potassium persulfate A rela tively small amount of persulfate present in the Probably the most serious factor iscoating will maintain the ferricyanide in oxidized form for long periods of time. The exact mode of operation'of the stabilizer has not been completely determined. It seems probable, however, that its stabilizing action is due to the fact that it is a strong oxidizer and possesses sufficient stability so that it does not give up nascent oxygen rapidly but remains in the coating in the form of a strong oxidizer for a long period of time. It is possible, however, that other-factors may be present, such as a chemical reaction between the persulfate .and ferricyanide, and hence the present invention is not intended to be limited to any particular theory of action. The persulfates are in no sense equivalents of ordinary oxidizing agents;' for example, hypochlorites although equally strong oxidizers will not exert a stabilizing action on blue print coatings which endure for long periods of time.

The stabilizer of the present invention may be incorporated into a blue print solution in any suitable manner. Thus, it may be added directly to the solution when it is made up or it may be. incorporated with the ferricyanide, particularly when the latter is marketed in the form of a liquor, the cheapest form in which sodium ferricyanide is obtainable.

While it is possible to use extremely crude ferricyanides, it is desirable that when first prepared their contents of ferrocyanide be reduced to a low point which can be done cheaply with cheap oxidizing agents such as chlorine, hypochlorite, and the like. If the ferrocyanide content is not reduced to a very low point, persulfate is used up and less remains to stabilize the blue print coating. Persulfate is a comparatively expensive oxidizing agent and it is therefore economically desirable to use as little of the persulfate as possible and hence the ferricyanide employed is preferably relatively free from ferrocyanide although the invention is not limited to the use of such ferricyanides. When sodium ferricyanide liquor is employed, the most desirable method of incorporating the persulfate is to add it to the liquor when it is freshly made as this will stabilize the liquor so that even after considerable storage it is still'substantially free from ferrocyanide.

The present invention is not limited to any particular critical amounts of persulfate. In general, from about 0.2-0.3 gram per 100 cc. of blue print coating solution will give excellent results. Smaller amounts of persulfate are effective, but the stability of the solution is less if the amount is much below 0.2 gram per 100 though we do'not limit our invention to this.

preferred amount.

The invention will be described in connection with the following specific examples which show the use of the features of the invention in connection with typical blue print coating solutions. Parts are by weight except in the case of liquids where they are by corresponding volume.

Example 1 226 parts of water are used to dissolve 24.9 parts of ferric ammonium oxalate and 24 parts of a sodium ferricyanide liquor containing approximately 215 parts of sodium ferricyanide per liter. To the ferricyanide liquor is added from 0.2-0.3 part of ammonium persulfate prior to the mixture of the ferricyanide liquor with the ferric ammonium oxalate.

The coating solution obtained which is light green-brown in color is coated onto paper in the usual manner and produces a blue print paper which is lemon yellow in color. .The pH of the coating solution is about 4.9.

A comparison test was made in order to determine the stability of the paper coated as described above. For this purpose small portions of the paper were tested every day by exposing and printing a small test strip. Backgrounds remained perfectly white up to 48 days when a very slight blue tone appeared. After 62 days the background appeared only slightly blue and therefore the commercial life of the paper was at least this long. Paper was coated with a coating solution prepared exactly as described above but without the ammonium persulfate. This paper had a. useful life of only 14 days. A third test was made with a still more crude ferricyanide liquor which contained considerable amounts of ferrocyanide. This paper had a useful life of about 6 days. Standard commercial blue print papers showed lives from slightly over two weeks to somewhat over a month, depending on the make and grade of the paper.

Another coating solution was prepared using potassium persulfate instead of ammonium persulfate. The results were practically identical. Ammonium persulfate is somewhat cheaper than potassium persulfate and is therefore to be preferred as it gives at least as good results as the more expensive potassium compound.

Example 2 A blue print coating solution was preparedby dissolving 24.9 parts of ferric ammonium oxalate in 100 parts of water, adding 5.07 parts ofpotassium ferricyanide and 0.2-0.3 part of ammonium persulfate or potassium persulfate. The solution was green-brown in color and had a pH of about 4.9. When coated onto paper, a lemon yellow color was obtained.

What we claim is:

1. A blue print coating solution containing a light sensitive iron, salt, a ferricyanide, and a small amount of a persulfate.

2. A blue print coating solution containing a light sensitive iron salt, sodium ferricyanide, and a small amount of a persulfate.

3. A blue print coating'solution containing a light sensitive iron salt, a ferricyanide, and a small amount of ammonium persulfate.

4. A blue print coating solution containing a light sensitive iron salt, sodium ferricyanide, and a small amount of ammonium persulfate.

5. A blue print coating solution containing a light sensitive iron salt, a ferricyanide, and 0.2%- l% of a persulfate.

6. A blue print coating solution containing a 

